To build all three portions of the traps, do exercises for each... not just shrugs. Here's how.

Know the anatomy of the traps and you'll know the best and most effective exercises. The trapezius is made up of three regions. Here's what they are and what they do:

Upper – Shrug shoulders upwards

Middle – Squeeze shoulder blades together

Lower – Bring shoulder blades downwards

The upper region originates on the back of the skull (occipital bone) and connects along the spine and ligaments of the vertebrae of the neck. It then inserts on the outer part of the clavicle (collar bone). The upper trapezius works by:

The middle and lower trap originates along the mid spine and inserts along the outer to inner part of the shoulder blades (spine of the scapula). The lower trapezius works by:

Retracting the scapula. Think of squeezing the shoulder blades together during seated rows.

Lowering the shoulders or scapular depression. Think of a reverse shrug using a lat pulldown.

Muscle Fiber Makeup

Another point to consider is the muscle fiber makeup of the traps. They're made up of 45% fast twitch and 55% slow twitch muscle fibers. This means they should be trained with both heavy weight and low reps as well as lighter weight and high reps.

Half the time you should be training traps with 6-12 reps at 60-80% of your 1RM. The other half you should be training traps with 12-30 reps at 50-60% of your 1RM (to near failure).

Finally, regarding where trap activation is greatest on a deadlift, if you look at the deadlift as three different parts you'll see the beginning (floor to mid-shin), middle (mid-shin to mid-thigh) and end phase (mid-thigh to lock out). Research shows that upper traps are maximally activated during the middle phase of the lift. So rack pulls starting at the mid-thigh are a good addition to target peak upper trap activation.